In electrophotography, an image comprising an electrostatic field pattern, usually of nonuniform strength (also retorted to as an electrostatic latent image), is formed on an insulative surface of a photoconductor drum. The insulative surface of the drum comprises a photoconductive layer and an electrically-conductive substrate. The electrostatic latent image may be formed by imagewise photo-induced dissipation of portions of an electrostatic field previously formed on the insulative surface of the drum. Typically, the electrostatic latent image on the drum is then developed into a toner image by contacting the latent image with an electrostatically-charged toner composition generally containing a pigment. The electrostatically-charged toner is generally disposed on the drum from a developer roller. In some systems, the toner image is then transferred with an intermediate transfer roller from the drum onto a transfer medium, such as paper, and fixed thereon by heating and/or pressure. The last step involves cleaning residual toner from the photoconductor drum. The rollers used in the development process are the developer roller and the transfer roller. These rollers require specific physical properties such as low dielectric constant, hardness, pliability, and long-term stability in a electrical, high-temperature, high-humidity environment. It is also desired that the rollers have an operating resistivity of about 5.times.10.sup.8 to about 5.times.10.sup.9 ohm-cm under operating temperature and humidity conditions.
Polymeric materials having suitable mechanical properties for use in development and transfer rollers are silicones, polysiloxane, and polyurethanes. Unfortunately, these materials each have too high of an electrical resistivity (e.g., 10.sup.11 to 10.sup.12 ohm.cm) to be suitable for use in electrophotographic development.
Kameda et al., U.S. Pat. No. 4,066,352, disclose a transfer roller for electrophotographic printing comprising a polyurethane foam covering on a rigid substrate. The polymeric covering has electroconductive powders dispersed therein to achieve the proper electrical resistivity. Kameda discloses as electroconductive powders such materials as salts, carbon black, and metal powders such as aluminum, silver, and the like. Unfortunately, the poor solubility and dispersability of the materials in the polymer results in nonuniform electrical resistivity. Further, certain dopants, such as carbon black, require high loading which negatively effects the mechanical properties of the covering. The salts are moisture sensitive and therefore cause large variations in resistivity and thus performance with changes in humidity.
Therefore, it is an object of the present invention to provide an improved roller for use in electrophotographic development. Other objects and advantages will be apparent from the following disclosure.